Previous studies using molecular hybridization of polysomal poly(A)-containing messenger RNAs of undifferentiated (S) and differentiated (P) mouse neuroblastoma cells with their complementary DNAs (cDNAs) had shown that P cells contained some messenger RNA sequences not present in S cells. After further refinement of the hybridization, it became clear that there were sequences specific to both S cells and P cells. The remainder of the year was devoted to attempts to clone, using recombinant DNA techniques (IMUA #108), these differentiation-step-specific sequences.